1. Field of the Invention
The present invention relates to an apparatus and method for real-time analysis of 1) trap gas, 2) mud fluid and/or 3) cuttings for gas content in conjunction with exploring the earth's subsurface for economic, producible hydrocarbons. In another aspect, the present invention relates to mapping the distribution, chemistry and relative and/or absolute abundance of chemical species analyzed by the above apparatus and method.
2. Prior Art
Petroleum resources are the cumulative result of generation, expulsion, migration and trapping of petroleum in sedimentary basins. Petroleum fluids (both gas and liquid) are retained in the source rocks and along migration pathways as residual petroleum saturation in macro or micropores during movement of these fluids from source to reservoir. Microscopic amounts of migrating or reservoired petroleum fluids are trapped within source rocks, along migration pathways or within petroleum reservoirs within healed fractures or porosity-occluding cements (i.e., fluid inclusions). Leakage or remigration of petroleum-bearing reservoirs can result in retained, non-economic petroleum residue within macro or microporosity in the reservoir sections. Finally, a given pore fluid may be substantially replaced by a subsequent fluid (hydrocarbon or aqueous) leaving little evidence of the prior fluid's presence, with the exception of fluid inclusions that are protected from alteration or displacement because they are completely encapsulated in mineral matter. This latter situation might exist, for instance, when a prior charge of oil is displaced by a later gas charge, due to density differences. In addition to the organic-dominated fluids mentioned above, natural inorganic species, such as CO2, He, Ar, N2, H2S, COS and CS2 are indicative of processes operative in the subsurface that are important to locating, understanding and exploiting petroleum occurrences.
It is known to circulate and analyze drilling fluid. Drilling fluid is generally circulated down a drill string to the bottom of a well. The drilling fluid is recovered from the well via a mud return line.
Current well site mudlogging operations generally include a device that analyzes gases emanating from the mud system circulated through the borehole during drilling. Generally the apparatus consists of a combustible gas detector (also known as a total gas detector or hot-wire detector) and, also, a gas chromatograph (GC) that typically analyzes alkanes with 1 to 5 carbon atoms. The total gas detector provides a more-or-less continuous record, while the GC operates on a cycle of 3–6 minutes. The gases that are detected represent some combination of pore fluids released from the volume of rock comminuted by the drill bit, fluids invading the borehole from formations that are overpressured with respect to the mud column, fluids generated through thermal processes at the drill bit (e.g., some so-called shale gases) and fluids derived from materials added to the mud system for a variety of reasons. Henceforth, these fluids are called borehole volatiles, while loosely or tightly encapsulated fluids within rock material are henceforth called cuttings volatiles regardless of whether they are derived from drill cuttings or drill core.
The systematic and comprehensive analysis of borehole volatiles and cuttings volatiles can be used to evaluate where petroleum fluids are currently, where they have been in the past, the composition and quality of petroleum fluids and other information useful to the oil and gas industry and particularly to well drilling and completion operations. Current methods provide a very incomplete record of above-described subterranean fluid history recorded by borehole and cuttings volatiles, due to the industry-standard choice of instrumentation and methodology. Specifically, the so-called hot-wire or total-gas detector provides only a measure of the total amount of combustible hydrocarbons without any compound specificity. Analysis of a split of these gases with a GC provides a measure of methane, ethane, propane, n-butane and iso-butane. Higher paraffins may be measured, but are not commonly. Limitations of this analysis stem from the fact that these species are all of the same class of hydrocarbon compounds (paraffins), hence, tend to react similarly to subsurface processes. The other two dominant classes of hydrocarbon compounds, naphthenes and aromatics are not explicitly analyzed. The relative distribution of these compounds can vary by several orders of magnitude in response to source rock attributes, migration processes and phenomena operative in the reservoir. While it is true that dry gas can be distinguished from wet gas or oil with well site gas detection equipment, it is difficult to distinguish between wet gas, condensate and oil with current GC based instrumentation. Ratios of low molecular weight paraffins are used in attempts to distinguish oil from gas (e.g., wetness factors), but these are often inadequate for the task.
It is not possible with GC-based methods to distinguish compounds that exist as a free phase in the pore system from those that may be dissolved in an aqueous pore fluid since GC methods generally do not measure a wide range of carbon species. This limitation prevents, for instance, distinguishing petroliferous formations from underlying water legs or water-bearing formations that are charged up dip, based on concentrations of water-soluble compounds such as benzene and acetic acid. Currently fluid contacts are identified solely based on decreases in paraffin gas abundance. The methodology and apparatus recommended herein provides evidence for petroleum-water contacts based on decreases in relatively water-insoluble compounds and concomitant increases in relatively water-soluble compounds.
Another critical element is the speed at which compounds can be collected. Although hot wire analysis is more-or-less continuous, typical GC cycle times are on the order of 3–6 minutes. Under fast drilling rates, this can translate to a sample analysis every 5 feet or more. Hence, thinly bedded pay horizons may be missed, or only recorded by an increase in total gas. The mass spectrometry based technique of this invention allows continuous monitoring of the gas flow, and cycle times as fast as 15 seconds. Even at slower times (up to 6 minutes), monitoring is continuous, so that an increase in borehole gas will be recorded almost instantaneously over the remaining mass range that is being scanned. The scan rate can be selected from the computer interface and implemented more or less instantly to fit the drilling rates anticipated, another feature that is not possible with a GC without extensive instrument modification.
Current art teaches away from using mass spectrometry (MS) on wellsite because of a perceived lack of reliability due to rugged conditions encountered in the field. The present design has been demonstrated to be more reliable than current GC technology, and less prone to operator error.
Prior art methods for analysis of fluid inclusions from a plurality of rock samples and stratigraphically mapping these chemistries are known (e.g., U.S. Pat. No. 5,286,651), however, that methodology and apparatus has some critical limitations that are improved upon by the current invention. First, previous methods advocate use of multiple mass spectrometers, whereas the preferred embodiment of the present invention can acquire substantially similar information with one mass spectrometer. In addition to cost savings, this obviates the need for inter-mass spectrometer calibration, and prevents analytical artifacts introduced by the unavoidable differences in sensitivity, resolution and the like, among mass spectrometers. Second, prior art teaches the advantage of jump scanning from mass to mass, whereas the current invention has found that continuous scanning allows more accurate peak location and better analytical statistics. Third, multiple scans, and specifically a large number of scans are advocated by prior art, however, it has been learned that the advocated procedure of jump scanning coupled with fast scan rates to get an abundance of scans in the time frame required, produces poor mass resolution due to recovery limitations of the electronics and decreases overall sensitivity because of poor counting statistics. Using few scans, slower scan speeds and continuous scanning mode produces much better precision, resolution and sensitivity. Finally, prior art involves placing multiple samples contained within multiple sample chambers in the same vacuum system and sequentially crushing them allowing the evolved gases from one sample to contact the surfaces of previous samples as well as those not yet analyzed. This procedure has several disadvantages, including potential cross contamination of samples and/or volatiles, development of progressively higher backgrounds during analysis of large sample sets unless unrealistically long pump-down times are employed between each sample, and selective near-instantaneous adsorption of released volatiles onto the surfaces of all samples in the chamber, resulting in fractionated and muted responses. Additionally, trace residual natural organic compounds, if present on grain surfaces, are additively contributed to the background and can create a disproportionately high background, which affects the baseline sensitivity of the analysis. It is advocated in prior art that this surface contamination be removed as much as possible, using vacuum heating and/or solvent extraction procedures. The current invention demonstrates the value of analyzing these trace natural surface organic species before removal and/or crush analysis of the trapped fluids. The resulting information can be used with borehole fluid analysis to distinguish among current charge in reservoirs, breached reservoirs, heavy oil or tar occurrences near oil-water contacts and migration pathways that have never accumulated significant oil saturation.
Other prior art approaches of analysis of gas content may be seen in Crownover, U.S. Pat. No. 4,635,735, wherein spectrophotometers utilizing a light signal are used for gas analysis.
While attempts have been made to improve some aspects of well site hydrocarbon detection (e.g., Quantitative Fluorescence Technique (QFT), Quantitative Gas Analysis (QGA), membrane technology), there is currently no comprehensive apparatus for analyzing past and present pore fluids in the necessary detail. Much information on current pore fluids at a given depth is lost once the borehole is drilled past that depth; hence, a portable apparatus capable of operating in a well site environment and functional for analyzing these fluids in real time is required. Cuttings volatile analysis can be completed on archived samples, but the surface adsorbed portion of the signal, discussed above, as well as the real-time application to drilling and completion operations are lost. For discussion purposes, real-time analysis refers to capability of analyzing samples shortly after they emanate from the well bore, generally within minutes to perhaps 1 hour.
In summary, the present invention relates to a method and apparatus for determining the composition of borehole volatiles and cuttings volatiles, which provide an adequate record of most of the natural volatile elements and compounds found in the subsurface, or added to the well bore by drilling personnel during drilling operations.
The invention also relates to compositional mapping of cuttings and borehole volatiles derived from the subsurface, and oil and gas exploration using the results of such analyses.